1. Field of the Invention
This invention relates to a height measurement apparatus for determining volume/density of wood chips on a conveyor. Such structures of this type, generally, employ the use of contacts that ride on top of the pile of wood chips in order to more accurately measure the volume and density of the wood chips.
2. Description of the Related Art
A conventional paper mill approach to measuring chip moisture relies on accurate, but slow, oven-drying of random belt grab samples of chips. Typically, chip moistures are measured several times throughout a day. This daily data becomes a part of monthly moisture averages that guides the paper mill digesting process. Consequently, this approach creates varying minimums and maximums throughout the year. Also, different wood species, different historical moisture and different sizes of chips available from chippers will alter the volume or density of a given chip pile. It is important to be able to accurately assess the chip moisture going into the digester, whether it a continuous or batch digester, of a paper mill because this affects the amount of chemicals that go into the digester and the operating conditions of the digester.
It is known to employ devices to correct flows of various items for moisture. Exemplary of such prior art is U.S. Pat. No. 3,186,596 ('596) to C. O. Badgett, entitled "Concrete Batch Blending Control System". While the '596 reference teaches how to correct a flow for moisture, it is not concerned with chip moisture for digesting. Also, it must be pointed out a similar negative result occurs for cement when moisture is unknown, namely the resulting product (concrete) quality can be poor. Finally, the '596 reference does not consider any impact of volume when measuring moisture.
It is also known to employ various devices for feeding wood chips into a digester. Exemplary of such prior art is U.S. Pat. No. 5,266,159 ('159) to B. Greenwood et al., entitled "Mass Flow Measurement, Preferably for Controlling Chip Feed to a Digester." While the '159 reference involves feeding a continuous digester with chips from a conveyor belt and controlling chip density, it recommends determining chip density in a free-fall of the chips. However, the '159 reference does not teach that volume of chips is important for density nor does it present a chip volume-determining method.
Several other references mention that both density and volume can be important when filling a container. Exemplary of such prior art are U.S. Pat. No. 4,582,992 ('992) to T. L. Atwell et al., entitled "Self-Contained, On-Line, Real-Time Bulk Material Analyzer," U.S. Pat. No. 3,822,032 ('032) to R. W. Vergobbi, entitled "Apparatus For Filling Containers Including Means Responsive to Both the Weight and the Height of the Material Dispensed," and U.S. Pat. No. 4,520,883 ('883) to M. Fukuda, entitled "Combinatorial Weighing Method and Apparatus With Volume." While these references mention both volume and density, the '292 reference uses gamma and neutron detectors to determine the "elemental content" of a bulk material in a chute based on the prompt gamma ray neutron activation analysis method. Similarly, the '032 reference employs a hopper that holds a preset allowable weight limit and a filling volume or height for a container. Finally, the '833 reference uses a consumer-product filling machine having a preset allowable weight limit and a filling volume or height for a container.
It is also known to employ a nuclear source for use in chip moisture measurement. Moisture is measured by such neutron devices by determining the hydrogen content of both the wood chip material and the conveyor belt, subtracting the hydrogen content of the belt and surroundings, and distinguishing between higher hydrogen contents due to more chip moisture or more chips in the pile. Exemplary of such prior art are U.S. Pat. No. 3,748,473 ('473) to Y. M. Chen, entitled "Gauge for Determining the Percentage by Weight of Moisture Contained in a Bulk Material Transported on a Moving Conveyor," U.S. Pat. No. 3,761,712 ('712) to P. L. Listerman, entitled "Gauge for Determining the Percentage by Weight of Moisture Contained in a Bulk Material Transported on a Moving Conveyor," and U.S. Pat. No. 3,794,843 ('843) to Y. M. Chen, entitled "Gauge for Determining the Percentage by Weight of Moisture Contained in a Bulk Material Transported on a Moving Conveyor." While the '473, '712, and '843 references employ a nuclear source for chip moisture measurement, the percent moisture is determined as the ratio of weight of water over total water. The weight of water is estimated by sensing slowed neutrons. The total weight is similarly determined by sensing the result of impacts of an object by gamma rays. The specifics of signal collection during the set of testing intervals is defined in the '712 reference. The special case of determining percent moisture for very high moisture levels and/or high levels of iron or sodium chloride is set forth in the '843 reference. However, none of these references consider correcting the radiation measurements for the volume of material being measured. Also, none of these references measure the volume or height.
It is also known to employ a chip moisture meter to measure chip volume on a conveyor. See, for example, FIG. 1. As shown in FIG. 1, the wood chips are transported down a conveyor. As the chips are transported down the conveyor, they are rearranged on the belt so as to have a Gaussian distribution cross-section of bulk material including a maximum height. An ultrasonic beam is used in the height sensor. While this device can accurately measure the height of the pile, it cannot accurately measure the density of the pile because the volume it estimates is not necessarily accurate. Also, this prior art device requires frequent re-calibration since chip distribution can vary with height.
It is apparent from the above that there exists a need in the art for an accurate height measurement system which is capable of measuring the volume and density of wood chips on a conveyor. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.